program cifra_III;
var a,b,c,k,cop,i,n,sol,pow:longint;
    p:real;
    f,g:text;
    cond:boolean;
begin
assign (f,'cifre.in'); reset (f);
assign (g,'cifre.out'); rewrite (g);
read (f,a,b,c,k);
sol:=0;
pow:=1;
for i:=1 to k-1 do pow:=pow*10;
for i:=a to b do begin cop:=i;
                       cond:=false;
                       n:=0;
                       while (cop>0) and (cond<>true) and (i div pow>=k) do begin if (cop mod 10=c) then n:=n+1;
                                                                                  if (k=n) then begin sol:=sol+1;
                                                                                                      cond:=true;
                                                                                                end;
                                                                                  cop:=cop div 10;

                                                                            end;
                 end;
p:=sol/(b-a+1);
write (g,p:4:4);
close (f);
close (g);
end.